The present invention relates to marine propulsion systems having an outboard two cycle internal combustion engine and a remote fuel tank, and more particularly to the fuel delivery system therefore.
The invention arose during development efforts directed toward solving a problem occuring as fuel runs out in systems using an automatic oil-fuel mixer, sometimes referred to as an autoblend unit, for example as shown in U.S. Pat. No. 4,583,500, incorporated herein by reference. The mixer draws fuel from a fuel tank and oil from an oil tank in a given ratio, typically about 50 parts fuel to 1 part oil, and automatically mixes the fuel and oil, eliminating the need to premix same. The mixer has a fuel inlet, an oil inlet, and an oil-fuel outlet, and is operated by a pressure differential between the fuel inlet and the oil-fuel outlet. The mixer may be operated by various sources of pressure differential, for example the mixer may be connected downstream of the fuel pump and use fuel pressure to operate the mixer. In other applications where a pressurized fuel tank is utilized, such pressure may be used to operate the mixer. In other implementations, crankcase pressure and/or vacuum may be used to operate the mixer, or a separate dedicated small pump may be used. In another implementation, the mixer is connected upstream of the fuel pump such that fuel pump suction on the oil-fuel outlet operates the mixer. In all the implementations, a problem of an overly rich oil-fuel mixture being supplied to the engine arises as fuel runs out.
In the last noted implementation, when the fuel tank runs out of fuel, then air from the tank is sucked through the mixer by the fuel pump. This air flow through the mixer continues to operate the mixer and causes continued delivery of oil from the oil tank to the engine, which in turn causes excess oil in the oil-fuel mixture as the fuel runs out, i.e. there is an overly rich oil-fuel mixture supplied to the engine from the remaining oil-fuel mixture in a carburetor bowl or the like as the latter runs dry.
The present invention addresses and solves the noted problem. Another solution is shown in copending application Ser. No. 07/218,310, filed on even date herewith, entitled "Automatic Oil-Fuel Mixer With Auxiliary Chamber".
It has also been found that the present invention enhances accuracy of the oil-fuel mixture ratio. In order to provide an accurate mix, the fuel must be in liquid form. If the fuel is in the form of vapor or if there are vapor or air bubbles in the fuel, then such air or vapor will displace the fuel in the mixer, and the oil-fuel mixture ratio will not be accurate. In the worst case, when only air is being drawn through the mixer as above noted when fuel runs out, then the mixer will provide all oil and no fuel. The present invention ensures that only fuel in the liquid form is mixed with the oil in the mixer, to maintain accuracy of the mixture ratio.